The development of coated papers has been marked by innovations relating to method and apparatus of applying liquid coatings as a film to a moving web; by the systematic exploration of possible coating formulations; and by the development of various techniques to dry the coating applied.
A crucial consideration in this effort has been the base stock to be coated and here effort has focused on existing techniques for modifying the characteristics of paper to optimize the operation. With the substantial technical advances that have been made in the other areas of coating methodology, at this point it might be observed that base stock preparation is the crucial area needing consideration.
Coatings are applied to paper or paperboard for a wide variety of purposes. For publication grades of paper or paperboard, enhancement of surface appearance is achieved by formuations using a pigment, a binder, and water. Other agents may be added for brightness, enhanced flow properties, and prevention of foam.
For boxboard, multiwall bag, and some other paper grades, a plastic coating such as polyethylene coating is widely employed to provide a measure of release, gloss, water repellency, and inhibition of moisture vapor transmission. Waxes are sometimes employed for similar purposes and to provide rigid-when-wet characteristics for corrugated containers.
Some specialty coatings are silicones, applied as water based emulsions, dissolved in solvents, or 100% solids; coatings of carbonless solution; zinc oxide coatings; and others which serve particular functions such as imparting release, image transfer under pressure, and electrostatic image transfer respectively.
There are an enormous variety of coatings. However, these present very similar problems in terms of base stock requirements. One essential base stock characteristic is evenness of moisture profile. A sheet with uneven moisture content will tend to accept coating unevenly. Another essential base stock characteristic is adequate wet strength. A sheet with insufficient wet strength, will weaken as the web contacts a liquid coating, and may break causing costly downtime and waste. Another essential base stock characteristic is capability of bonding to the coating solution. This bond is a key factor in producing a durable coating. Finally, perhaps the most essential base stock characteristic is holdout to the coating solution. The rate and extent to which the base stock absorbs the coating solution affects pickup of the coating, sheet breaks, drying, speed of operation, and quality of finished product.
The interaction between the base stock and the coating solution is the heart of the coating process. An inadequacy of base stock can sometimes be adjusted through modification of the coating solution, method, or both. The direction of approach in solving these problems is to some degree dependent on whether the fibrous component of the final product is more costly on a weight basis or whether the coating component is the more costly. As the final product will, in most cases be sold by weight, much research and development effort has been focused on enhancement of technique to achieve reduction of fibrous component or coating component without mitigating essential characteristics of the final product.